To prevent rechargeable battery packs from being damaged, and therefore prolong their service life, conventionally a parameter connected with the charging condition of such battery packs is detected with a view to stopping the charging, at least at a high level, as soon as a predetermined level of charge is reached. The continuation of charging beyond this predetermined charging level may have serious disadvantages, more particularly, in the case of sealed battery cells such as, for example, cadmium-nickel cells.
One of the conventional parameters for detecting the predetermined level of charge is the voltage across the terminals of the battery pack. Devices sensitive to such a parameter are disclosed, for example, in French Pat. No. 1,436,908.
Another parameter for detecting the predetermined level of charge is the temperature of the rechargeable battery cells, due to the heat evolved by the chemical reaction produced in the battery cells at the end of the charging process. Devices sensitive to this second parameter are disclosed in French Pat. Nos. 1 366 883 and 1 572 057.
Devices for charging a battery pack which are sensitive to the temperature of the battery pack in a general way incorporate a temperature-sensitive element, such as a thermistor, which must be brought into contact with the battery pack.
A technique used for this purpose consists of disposing the thermistor during charging in a suitable recess in the battery pack, in a position adjacent to one of its cells. However, this technique is unsatisfactory in practice, since although the thermistor is disposed adjacent the battery cell, it is not brought into contact with such battery cell. The thermistor, therefore, fails to detect the real temperature of the cells of the battery pack, but detects an ambient temperature adjacent such cells.
This leads to occasional over-charging of the pack, inevitably causing its deterioration.
It will moreover be noted that this technique demands action on behalf of the operator. Consequently, in cases in which the operator forgets to position the thermistor on the battery pack, the charging may be continued until the battery pack is completely destroyed.
An attempt has been made to obviate this disadvantage by incorporating the thermistor in the structure of the battery pack during its manufacture. This in a general way ensures that the temperature of the battery pack will be detected during the charging process. However, this also requires a correct connection to be definitely established between the charging device and the thermistor. Otherwise, the charging accepts wrong temperature information and the result may be disastrous. Further, the incorporation of the thermistor in the structure of the battery pack greatly complicates the process of manufacturing such packs and, on the other hand, makes them substantially more expensive.
Moreover, in practice it is found that the electric connections of the temperature-sensitive element or that element itself sometimes deteriorates when the tool or device incorporating the battery pack is used. This is more particularly the case with portable tools subject to vibrations.